It is generally known in the art of internal combustion engine design to use tumble control valves in an intake manifold. “Tumble” is understood to mean air turbulence about an axis perpendicular to the longitudinal axis of the cylinder. The opposite of tumble is known as “swirl” which is air turbulence rotating about an axis generally parallel to the cylinder axis. Thus, tumble control valves work, during certain selected engine conditions, to induce tumble to the air flow as the air flow enters a cylinder chamber. It has been shown that tumbling of the air flow improves the homogeneity of the fuel-air mixture which will thus burn more effectively. Tumble is particularly desirable and effective during the first 20 seconds of a cold engine start and also at light engine load conditions (e.g., less than about 3,000 rpm).
Prior art tumble control valves generally comprise a valve body having an internal cavity and a valve flap or blade pivotally disposed within the internal cavity of the valve body. The valve blade pivots between fully open and fully closed positions. When in the fully closed position, the blade lies substantially perpendicular to the longitudinal axis of the valve body. In this position, the bottom edge of the blade lies in close relationship to the bottom wall of the internal cavity, thus effectively sealing off this area to air flow. Conversely, the top edge of the blade lies in spaced relation to the top wall of the internal cavity to define a gap wherethrough air may flow. Thus, in the fully closed position of the valve blade, maximum tumble is generated since the air flow is forced to pass through the gap defined along the top wall of the internal cavity of the valve body. In these prior art tumble control valves, as soon as the blade begins to pivot from the fully closed position, another gap immediately opens between the blade bottom edge and the internal cavity bottom wall. This is due to the fact that the distance between the blade bottom edge and the internal cavity bottom wall increases as the blade is pivoted toward the open position. This gap thus becomes continuously larger as the blade pivots to the fully open position. It will be appreciated that once this bottom gap opens, air will flow through it as well as the top gap and the amount of tumble will be reduced accordingly. While tumble is not needed or intended to be produced in the fully open position of the valve, a reduction in tumble is generally undesirable through small rotations of the blade from the fully closed position. For example, manufacturing tolerances often are not tight enough to consistently and reliably cause the valve to be in the fully closed position when it should be (e.g., at cold start of engine). Thus, in conditions where the valve should by fully closed, the valve blade may have unintentionally rotated away from the fully closed position by several degrees. Should this occur, a bottom gap forms and tumble is thereby reduced from its maximum potential as described above. Fuel efficiency and emissions control thereby also suffer, an undesirable consequence.